Optical Cable and Method for Producing an Optical Cable

ABSTRACT

An optical cable comprises a tight-buffered optical cable and a protective sleeve which surrounds the tight-buffered optical cable. An intermediate layer surrounds the protective sleeve has tension-resistant elements. Furthermore, the optical cable contains a cable sheath which surrounds the intermediate layer, and a transitional area facing its inner surface. In this transitional area, the material of the cable sheath is mixed with the tension-resistant elements of the intermediate layer.

RELATED APPLICATION

This application claims priority under 35 U.S.C. 119(a) to GermanApplication No. 102008015605.1, filed Mar. 26, 2008.

TECHNICAL FIELD

The present disclosure relates to an optical cable which is particularlysuitable for various indoor and outdoor applications. The disclosurealso relates to a method for producing an optical cable such as this.

BACKGROUND

The increasing network and the increasingly stringent requirements fordata transmission even in the private domestic field are leading to useof optical cables. In addition to regional distribution, the so-calledlast mile is also becoming increasingly important, where an opticalcable connects a distribution station to individual buildings ordwellings. In this case, the primary factor is to use special cableswhich can be used without any additional measures both within a buildingand outside a building since this avoids spliced joints or plugconnections which are susceptible to faults. So-called “drop cables”such as these can be laid inside and outside buildings, and outside theground or else as underground cables there.

Cables with different application ranges such as these are intended tosatisfy a multiplicity of requirements. On the one hand, the cableshould be as light and small as possible in order to allow it to be laidand processed further without any difficulties. Furthermore, it shouldhave as tight a bending radius as possible in order also to be laid witha bending radius of down to 20 mm within buildings. Because of the useof these cables within buildings, the materials used must comply withthe appropriate fire protection regulations. Furthermore, it should bepossible for the cables to be already provided with plug connectors, inorder to speed up the installation process, as prefabricated cable.

A cable such as this for the stated application ranges is subject towidely differing environmental influences. In particular, the resultanttemperature differences between a heated building and the outdoor area,which may be considerably cooler, leads to different expansion of theoptical cable, as a result of which the optical fiber may be subject totension loads. One reason for this is the different materials from whicha cable such as this is formed. Their expansion behavior can leadindirectly to a change in the attenuation, thus possibly adverselyaffecting the data transmission rate. In the worst case, for example,the optical fiber can be drawn off completely in the area of a plugconnection.

SUMMARY

An optical cable and methods for the production thereof are disclosed inwhich the cable's shrinkage response is reduced so that it is suitablefor a multiplicity of applications inside and outside buildings.

In one refinement, the optical cable comprises a tight-buffered opticalcable and a protective sleeve which surrounds the tight-buffered opticalcable. An intermediate layer surrounds the protective sleeve and alsohas a plurality of tension-resistant elements. Finally, the opticalcable contains a cable sheath which surrounds the intermediate layer andhas a transitional area which faces its inner surface. In thetransitional area, the material of the cable sheath is mixed with thetension-resistant elements in the intermediate layer.

This ensures close contact between the cable sheath and the intermediatelayer, and the further inner sleeves of the optical cable, thus reducingany undesirable shrinkage process of individual components of theoptical cable. In other words, the cable is distinguished in part inthat it results in particularly pronounced mechanical coupling of thecable sheath to the protective sleeve, which is ensured in particular bythe manufacturing process described further below. The mechanicalcoupling between the cable sheath and the protective sleeve leads to amore uniform length change of the layers as a function of thetemperature.

In another embodiment, an optical cable comprises a tight-bufferedoptical cable and a protective sleeve which surrounds the tight-bufferedoptical cable forming a gap. An intermediate layer is also provided, isarranged around the protective sleeve and has tension-resistantelements. A cable sheath is arranged around the intermediate layer andis operatively connected via the intermediate layer to the protectivesleeve such that any relative shrinkage of the protective sleeve and thecable sheath after 24 hours with respect to the tight-buffered opticalcable is not greater than 3 mm to 5 mm over a cable length of 3 m, andat a temperature of approximately 80° C.

The components, in particular the protective sleeve and the cablesheath, are therefore firmly coupled to one another in the refinement,so that any different expansion behavior has only a minor effect on thetight-buffered optical cable, because of the strong mechanical coupling.

In one embodiment of the method for producing an optical cable, atight-buffered optical cable is, inter alia, provided and a protectivesleeve is extruded around it. The protective sleeve is surrounded by anintermediate layer which has tension-resistant elements. Furthermore, acable sheath is pressure-extruded around the intermediate layer thusresulting in a transitional area along the inner surface of the cablesheath, in which the material of the cable sheath is mixed with thematerial of the intermediate layer.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Various aspects and embodiments will be explained in detail in thefollowing text with the assistance of the drawings, in which:

FIG. 1 shows a graph to explain the shrinkage behavior of the protectivesleeve in the longitudinal direction of the cable, as a function of thecable soak time;

FIG. 2 shows a graph illustrating the lateral shrinkage behavior of anoptical cable as a function of the cable soak time;

FIG. 3 shows a cross-sectional illustration of one embodiment of anoptical cable based on the proposed principle;

FIG. 4 shows a schematic illustration of a production line in order toexplain the production method;

FIG. 5 shows a schematic illustration of extrusion heads in order toexplain different extrusion options.

The invention can be implemented in various ways independently of thedescribed embodiments, and is not restricted to the schematicillustrations. In fact, the figures and the associated description forma basis to explain the various aspects of the invention. The figureshave therefore not been drawn to scale, and in fact individual elementsare illustrated larger or smaller, for clarity reasons. Componentshaving the same effect and/or function have the same reference symbols.

DETAILED DESCRIPTION

FIG. 3 shows a schematic cross-sectional view of an optical cable basedon the proposed principle, in which the lateral shrinkage behavior ofthe individual layers is reduced because of the strong mechanicalcoupling between the layers. A cable such as this can therefore beprocessed further and, for example, can be provided with a plug,particularly easily.

In the illustrated embodiment, the optical cable comprises atight-buffered optical cable 40 which has a waveguide, for example anoptical filber. In the exemplary embodiment, this is illustrated as thefiber 43, in which a fiber core is surrounded by so-called cladding. Thecladding is a glass and has a different refractive index to that of thefiber core. In addition, the fiber 43 also contains a protectivecoating. The fiber 43 is firmly surrounded by a sheath material 42. Thesheath material 42 protects the optical fiber 43 and surrounds itcompletely, resting closely on it. The diameter of the tight-bufferedoptical cable 40 (TB) in this embodiment is about 900 μm. However, itmay also have other values, for example between 400 μm and 900 μm,depending on the size of the fiber core 43 and the material 42surrounding the core. The sheath 42 may be composed of a material basedon silicone, polyvinylchloride, polyester, polyuretane or else othermaterials.

A protective sleeve 20 is also arranged around the tight-bufferedoptical cable 40 and in the present exemplary embodiment has a diameteron its inner face 22 which is slightly larger than the correspondingexternal diameter of the tight-buffered optical cable 40. This resultsin an essentially annular intermediate space 30, which in the presentcase has a thickness in the region of 50 μm. In consequence, theinternal diameter of the protective sleeve 20 is in the region of 1000μm.

Polyethylene, polyurethane, polypropylene, polyvinylchloride,polybutylene or else polyvinylchloride or a combination thereof may beused, inter alia, as the material for the protective sleeve 20.Furthermore, polycarbonates and polycarbonate mixtures are suitable,since they have high stiffness. One example of a protective sleevematerial is a mixture of polycarbonates andacrylonitrile-butadiene-styrene (ABS).

The annular intermediate space or the gap 30 is filled with ahigh-viscosity gel 32, which completely fills the intermediate spacebetween the protective sleeve 20 and the tight-buffered optical cable40. In particular, a gel can be used as the filler material whichexhibits only little to no diffusion into the sheath material of thetight-buffered optical cable 40 and into the protective sleeve 20. Thisreduces damage to or ageing effects on the sheath material of thetight-buffered optical cable 40 and the protective sleeve 20.Furthermore, the viscosity at a temperature of 23° C. should not be lessthan 4000 mPas (milli-Pascal seconds), in order to achieve adequatemechanical coupling between the protective sleeve 20 and thetight-buffered optical cable 40. Viscosity values of greater than 6000mPas are expedient.

The resultant intermediate space 30 allows a certain amount of play whena bending load is applied to the optical cable, with the high viscosityof the gel that is present at the same time producing good mechanicalcoupling between the protective sleeve and the tight-buffered opticalcable. The gel therefore also matches the shrinkage behavior of theprotective sleeve 20 to the tight-buffered optical cable 40.

In this context, FIG. 1 shows a graph which illustrates the commonshrinkage behavior of the tight-buffered optical cable with theprotective sleeve in comparison to the protective sleeve without thetight-buffered optical cable 40, and therefore without the mechanicalcoupling between the two elements.

The soak time is plotted in minutes on the abscissa, and the shrinkagebehavior in millimeters on the ordinate. The measurements were carriedout at a temperature of 70° C. and with a cable length of about 30 cm.

As can be seen, the joint mechanical coupling of the protective sleeveto the tight-buffered optical cable (values: protective sleeve with TB)remains essentially close to zero irrespective of the soak time. Theabbreviation “TB” in the following text is short for tight-bufferedoptical cable. In contrast, the protective sleeve without mechanicalcoupling to the tight-buffered optical cable (values: protective sleevewithout TB) shrinks by about 0.5 mm after a soak time of approximately200 minutes, over a cable length of about 30 cm.

The filling compound of high viscosity therefore matches the shrinkagebehavior of the protective sleeve to the shrinkage behavior of thetight-buffered optical cable, so that the relative change between theprotective sleeve 20 and the tight-buffered optical cable 40 is verysmall.

The protective sleeve 20 is now surrounded by a layer 12 which containsat least one yarn composed of aramide fibers 14. Synthetic polyamides inwhich at least 85% of the amide groups are directly bonded to twoaromatic rings are referred to as aramides or else aromatic polyamides.

The synthetic fiber aramide is distinguished by high strength withrespect to a strain or tensile load, as well as resistance to acids andlyes. Furthermore, it is highly resistant to heat and fire, does notmelt, but starts to carbonize at about 400° C. The intermediate layer 12may contain woven aramide fibers and may additionally also haveglass-fiber elements. The intermediate layer 12 is used to reinforce theoptical cable, in particular for tensile loads, in order to prevent thecable core 43 of the tight-buffered optical cable from breaking ortearing. Apart from fibers composed of aramide, polyvinyl ketones andhigh molecular weight polyethylenes as well as fiberglass orcombinations of them may also be used for this purpose.

Finally, a cable sheath 10 is extruded around the intermediate layer 12with the aramide fibers 14. This is done by means of pressure extrusion,so that a transitional area 15 is formed on the inside of the sheathsurface. In the transitional area 15, the extruded sheath material ismixed with aramide fibers 14 of the outer surface of the intermediatelayer 12. This results in strong mechanical coupling between the cablesheath 10, via the intermediate layer 12, and the protective layer 20.The transitional area 15 in which the cable sheath material mixes withthe aramide fibers may have a thickness of two tenths of the thicknessof the extruded cable sheath.

In addition, a thread 13 is incorporated in the cable sheath 10 and isused to open and to remove the cable sheath before a splicing process,in order to splice the fiber core of the tight-buffered optical cable toan optical waveguide.

The cable sheath 10 is composed of a flame-retardant, non-corrosivematerial, which is also referred to as FRNC material. Polyethylene orelse a mixture with polyethylene and ethylenevinylacetate may be usedfor the cable sheath. By way of example, aluminum trihydroxide or elsemagnesium hydroxide are used as flame-retardant materials.

The pressure-extrusion process for producing the cable sheath leads tothe cable sheath having considerably less lateral mobility with respectto the protective sleeve 20, which is provided within the cable sheath,and the tight-buffered optical cable 40.

In this context, FIG. 2 shows an illustration of the shrinkage behaviorplotted against the soak time for two cables produced in different ways.The values annotated “standard” were measured with a conventionallyproduced cable, in which the cable sheath was not connected to thelayers located underneath it by means of the pressure-extrusion processas described in detail further below. The “standard” cable is soaked ata temperature of 70° C. After a soak time of about 100 minutes, theshrinkage of the cable sheath rises sharply over a cable length of 1 mand, for example, is 2 mm after a soak time of about 200 minutes. Incontrast to this, one produced using the proposed method has aconsiderably reduced shrinkage behavior, indicated by the values“optimized”. These values were determined with a cable based on theproposed production principle, in particular with pressure extrusion ofthe cable sheath in the conditions stated above.

Because of the mixing, the optical cable exhibits considerably lessshrinkage, because of the mechanical binding of the cable sheath to thelayers located underneath it, via the intermediate layer. The averageshrinkage value of the optical cable based on the proposed principle, inparticular of the cable sheath and the protective layer relative to thetight-buffered optical cable, is in the region of 5 mm over a cablelength of 3 m, which was soaked at 80° C. for 24 hours and was thenmeasured at room temperature. Conventionally produced cables exhibitshrinkage that is greater by a factor of 3 by comparison.

In other words, the mechanical coupling of the cable sheath to theprotective sleeve, and in turn between the protective sleeve and thetight-buffered optical cable, results in the relative length changes ofthe individual elements being matched to one another. The mechanicalcoupling therefore leads to a length change, for example because of ashrinkage process or because of a temperature change, affecting all theelements approximately uniformly.

A further aspect therefore also relates to the so-called fiber excesslength which is now no longer required, or is scarcely still required,because of the reduced and more uniform shrinkage behavior. The fiberexcess length, that is to say the fiber section of the optical fiberwhich is longer than the surrounding cable sheath or the surroundingprotective sleeve, can therefore be less than 0.1% of the overalllength. In particular, values of less than 0.05% down to 0% are alsopossible. The latter value means that the length of the fibercorresponds to the length of the protective sleeve and of the cablesheath.

A plug or a plug connection can be fitted to the end of the cable withthe tight-buffered optical cable exposed. This is attached to the cable,for example to the yarn of the intermediate layer for strain reliefFurthermore, it is connected directly to the tight-buffered opticalcable. This can now be done without major difficulties since the commonand at the same time small expansion of the entire cable prevents thefiber from breaking in the area of the plug. In particular, the cablecan therefore be supplied with a plug at the same time.

One alternative approach to determine the mechanical coupling betweenthe individual elements of the cable is to determine the so-calledwithdrawal force. In this case, the force is measured which is requiredin order to separate one element of the optical cable from otherelements at a specific withdrawal speed. For example, the withdrawalforce which is required to pull the fiber core 40 out of the protectivesleeve 20 is at least 0.7 N at a pulling speed of 40 to 50 mm/min. Thewithdrawal force is dependent on the filling material 32 used betweenthe protective sleeve 20 and the tight-buffered optical cable 40. Thefilling material 32 is preferably a gel which results in a withdrawalforce of 2 N to 3 N for a cable length of 1 m with a withdrawal speed of40 to 50 mm/min.

The withdrawal force can be determined in a corresponding manner betweenthe protective sleeve 20 and the cable sheath 10 which surrounds theprotective sleeve. This withdrawal force should be in the range from 20N to 60 N, and in particular in the range from 30 N to 60 N, for awithdrawal speed of 40 to 50 mm/min and a cable length of 1 m. Highwithdrawal forces such as these can be achieved in particular by thepressure extrusion that is used for production of the sheath around theintermediate layer 12 and the protective sleeve 20.

FIG. 4 shows a schematic illustration of a production line for producingan optical cable 10. The production line contains a plurality ofindividual production units V1, V2 and V3 which are arranged on behindthe other. A tight-buffered optical cable 210 is wound on to a spool C1and is fed to the first manufacturing unit V1. The tight-bufferedoptical cable 210 has a fiber core and a surrounding sheath materialcomposed of a polymer, and, for example, has a diameter of 500 μm.

The manufacturing unit V1 has a tank T1 which is connected via anextruder E1 to an extruder head CH1. The tank T1 is filled with anextruding material, which forms the raw material for the protectivesleeve 20. As an example, materials based on polycarbonates may be usedfor this purpose and, furthermore, may also have flame-retardantcomponents. Flame-retardant materials can therefore be provided in theraw material itself, or else may be mixed in the extruder materialduring the extrusion process.

Once the material has been heated in the extruder E1, the hot polymermelt is extruded around the tight-buffered optical cable 210 by means ofthe extruder head CH1. The extruder head CH1 is in this case set suchthat a narrow gap 30 is formed between the tight-buffered optical cable40 and the inside of the protective sleeve 20. The sleeving extrusionwhich is described in this case and will be described in more detaillater on, makes it possible to determine the gap size, which ispreferably in the region of a few tens of micrometers, in particular inthe region of 50 μm.

Furthermore, the manufacturing unit V1 has provision for a fillingmaterial to be introduced into the gap between the tight-bufferedoptical cable 40 and the protective sleeve 20. In the present exemplaryembodiment, the filing material is a high-viscosity gel with a viscosityin the range from 4000 mPas to 12000 mPas, measured at a temperature of23° C. In one particular embodiment, a material is used whose viscosityis at least 6000 mPas or greater, measured at 23° C. Alternatively,yarns or other materials can also be introduced into the protectivesleeve or into the gap, for support.

The extruded protective sleeve is cooled down in the production unit V1thus resulting in a stiff protective sleeve which is mechanically wellcoupled to the tight-buffered optical cable. The partially produced andcooled-down cable is then fed to the second production unit V2. This nowsurrounds the protective sleeve 20 with a yarn composed of high tensilestrength material 240, thus forming the intermediate layer 12 as shownin the exemplary embodiment in FIG. 3.

By way of example, aramide, polyvinylketone or else very long-chainpolyethylenes can be used for this purpose. In addition, the fiber-glassfibers can be interwoven for further mechanical robustness. Therobustness elements are preferably arranged symmetrically around theprotective sleeves 20, and are operatively connected to the protectivesleeve.

The further production unit V3 is connected to the production unit V2and comprises a second tank T2 which is connected via a second extruderE2 to a further extruder head CH3. The extruder head CH3 is designed forpressure extrusion.

The cable emerging from the production unit V2 is fed to the extruderhead CH3. At the same time, the sheath material in the tank T2 is heatedvia the extruder E2, and the hot melt is forced at high pressure in theextruder head CH3 around the cable passing through the extruder head.The melt is therefore brought into contact with the intermediate layerin the extruder head itself. The pressure extrusion process results inthe hot cable material being mixed with the aramide and/or glass fibersof the intermediate layer in a transitional area of the cable sheath,and leads to good mechanical coupling and to the better shrinkagebehavior.

The cable emerging from the extruder head CH3 is then cooled in a waterbath W and is wound up on the second roller C2. In this case, however,no further tensile load is applied, in contrast to sleeving extrusion,and, instead, the cable sheath is provided with its predetermined shapeessentially by the opening in the extruder head CH3. From the point ofview of a molecular structure, pressure extrusion in the extruder headCH3 reduces alignment of the polymer chains in the sheath material sothat the morphology of the sheath material comes closer to an unorientedequilibrium distribution. If necessary and desired, the cable is thenalso provided with a terminating plug.

In the illustrated exemplary embodiment, the production line iscontinuous, that is to say the cable is manufactured in a continuousprocess. However, it may be worthwhile interrupting the processillustrated in FIG. 4 in order to allow the cable or the cable core tocome to rest and if appropriate to wait for production-dependentshrinkage processes. For this purpose, by way of example, the cable canbe wound on to a further drum, which is not illustrated, and may bestored briefly after each production unit, in particular after theproduction unit V1. This allows the cable core, that is to say thetight-buffered optical cable surrounded by the protective sleeve, toassume its final shape after the first production step. Furthermore,this makes it possible to compensate for different production speedsbetween the individual production units V1, V2 and V3.

In this context, FIG. 5A shows a cross-sectional view of the extrusionhead CH3 for pressure extrusion. The extrusion head CH3 comprises anipple, which tapers conically, and is provided with an opening in thefront area. The diameter of the opening of the nipple correspondsessentially to the diameter of the cable core passing through,comprising the tight-buffered optical cable, the protective sleeve andthe intermediate layer. A mouth piece is fitted to the nipple and itsopening so that this results in a conically tapering feed for the sheathmaterial between the nipple and the mouth piece. The mouth piece isdesigned such that the sheath material which is introduced through thefeeds is pressed firmly on to the cable core and is guided jointly alonga section L as far as an opening O. The opening O of the mouth piece hasa diameter which corresponds essentially to the external diameter of thedesired cable. In other words, the sheath material is pressed on to thecable core in the area L within the mouth piece, thus resulting inmixing in the transitional area of the cable sheath. The cable emergingfrom the extruder head is then cooled in the water bath.

In contrast, FIG. 5B shows an exemplary embodiment for sleevingextrusion, as is used by way of example in the extruder head CH1. Anipple is likewise provided there, through which the cable is passed. Inthe present case, this is the tight-buffered optical cable. In contrastto the pressure-extrusion head CH3, the opening in the nipple also formsthe opening in the extruder head CH1. Furthermore, the material of theprotective sleeve is extruded via a further mouth piece, which is placedaround the nipple. In this case, the diameter of the annular opening ofthe mouth piece is designed such that the extruded material does notcome into direct contact with the cable core. The annular diameter ofthe opening of the mouth piece is designed such that the opening in thenipple is placed in the opening of the mouth piece.

In consequence, the protective sleeve material does not make contactwith the tight-buffered optical cable until after leaving the extrusionhead. This is done by the protective sleeve material and thetight-buffered optical cable being drawn along the illustratedZ-direction.

In the sleeving extrusion head CH1, the outlet opening of the nipple hasa slightly larger diameter than the tight-buffered optical cable. Thisis used to bring gel that has been introduced into the cavity H intocontact with the tight-buffered optical cable, so that thetight-buffered optical cable is coated with a thin gel layer. When thetight-buffered optical cable is then drawn through the nipple in thedirection of the opening, the gel forms a thin layer on the surface ofthe tight-buffered optical cable, and leaves the extruder head CH1 inthis way.

The material of the protective sleeve is then not applied directly tothe tight-buffered optical cable but to gel surrounding thetight-buffered optical cable. This results in the extruded protectivesleeve having a slightly larger internal diameter, thus forming a gapbetween the protective sleeve and the tight-buffered optical cable, inthe region of 50 μm, for example. This allows a small amount of play forthe tight-buffered optical cable within the protective sleeve, andprevents the tight-buffered optical cable from sticking to theprotective sleeve, thus resulting in undesirable attenuation. At thesame time, the tight-buffered optical cable is kept in a straight lineand is not curved within the protective sleeve.

A mixture of polycarbonates with acrylonitrile-butadiene-styrene can beused as the material for the protective sleeve, and can also be providedwith flame-retardant components. For example, in addition, aluminumtrihydroxide or magnesium hydroxide making up a proportion by weight ofup to 60% can be introduced into the matrix of the protective sleeve.This is also done in the area of the feeds to the extruder head, inwhich the cable sheath material is mixed with the flame-retardantcomponents.

The described cable design can be used as a “drop cable” for indoor andoutdoor applications. In this case, it complies with the size andflexibility requirements. The stiff protective sleeve protects thetight-buffered optical cable against external pressure and allows evensmall bending diameters to be achieved. Furthermore, the shrinkagebehavior of the individual layers is reduced because of the goodmechanical coupling of the layers to the tight-buffered optical cable.This allows the cable to be coupled directly to an appropriate plugconnection without any need to divide additional reinforcing elements oradapters. In this case, the tight-buffered optical cable and the stiffprotective sleeve likewise improve the connection to a plug.

The optical cable is more robust than the previously used connectingcables, and is at the same time more flexible to handle than previoussolutions. It can be used not only indoors but also outdoors in variousexternal conditions without the shrinkage behavior resulting inunacceptable and an additional optical attenuation.

1. An optical cable, comprising: a tight-buffered optical cable; aprotective sleeve surrounding the tight-buffered optical cable; anintermediate layer surrounding the protective sleeve and havingtension-resistant elements; and a cable sheath surrounding theintermediate layer and having a transitional area faces its innersurface and in which a material of the cable sheath is mixed with thetension-resistant elements of the intermediate layer, the mixing of thematerial of the cable sheath the the tension-resistant elements reducingtemperature-induced length changes, wherein the transitional areacomprises 0.1% to 20% of the thickness of the cable sheath.
 2. Theoptical cable of claim 1, wherein the transitional area comprises 10% to20% of a thickness of the cable sheath.
 3. The optical cable of claim 1,wherein the intermediate layer includes at least one of glass fibers anda yarn composed of aramide fibers.
 4. The optical cable of claim 1,wherein the material of the cable sheath comprises a flame-retardant,non-corrosive material.
 5. The optical cable of claim 4, wherein thematerial of the cable sheath has polyethylene and aluminum trihydroxideor polyethylene and magnesium hydroxide.
 6. The optical cable of claim1, further comprising a gap between the tight-buffered optical cable andthe protective sleeve, the gap having a thickness in the range from 30to 100 μm.
 7. The optical cable of claim 1, wherein the internaldiameter of the protective sleeve (20) is not greater than 1000 μm. 8.The optical cable of claim 1, wherein an external diameter of thetight-buffered optical cable is about 900 μm.
 9. The optical cable ofclaim 1, further comprising a a gel with a viscosity of greater than4000 mPas at a temperature of 23° C. located between the protectivesleeve and the tight-buffered optical cable.
 10. The optical cable ofclaim 9, wherein the viscosity of the gel is in the range of 6000 mPasto 12000 mPas at a temperature of 23° C.
 11. The optical cable of claim1, wherein the protective sleeve is operatively connected via theintermediate layer to the cable sheath such that any relative shrinkageof the protective sleeve and of the cable sheath with respect to thetight-buffered optical cable after 24 hours is not greater than 3 mm to5 mm over a cable length of 3 m and at a temperature of approximately80° C.
 12. The optical cable of claim 1, wherein the tight-bufferedoptical cable and the protective sleeve make contact with one anothersuch that a withdrawal force exerted on the tight-buffered optical cableor on the protective sleeve is not less than 0.7 N at room temperatureand with a withdrawal speed in the range from 40 mm/min to 50 mm/min,over a cable length of 1 m.
 13. The optical cable of claim 12, whereinthe withdrawal force is in the range of 2 N to 3 N.
 14. The opticalcable of claim 1, wherein the protective sleeve and the cable sheath areoperatively connected to one another via the intermediate layer suchthat a withdrawal force on the protective sleeve from the cable sheathis in the range of 30 N to 60 N at room temperature and at a withdrawalspeed in the range from 40 mm/min to 50 mm/min, over a cable length of 1m.
 15. An optical cable, comprising: a tight-buffered optical cable; aprotective sleeve surrounding the tight-buffered optical cable forming agap; an intermediate layer surrounding the protective sleeve and hastension-resistant elements; and a cable sheath around the intermediatelayer and operatively connected to the protective sleeve via theintermediate layer such that any relative shrinkage of the protectivesleeve and of the cable sheath with respect to the tight-bufferedoptical cable after 24 hours is not greater than 3 mm to 5 mm over acable length of 3 m and at a temperature of approximately 80° C.,wherein a withdrawal force of the protective sleeve from the cablesheath is in the range from 30 N to 60 N at room temperature and with awithdrawal speed in the range from 40 mm/min to 50 mm/min, over a cablelength of 1 m.
 16. The optical cable of claim 15, wherein thetension-resistant elements comprise aramide fibers.
 17. The opticalcable of claim 15, wherein a material of the cable sheath and thetension-resistant elements are are mixed in a transitional area betweenthe intermediate layer and the cable sheath.
 18. The optical cable ofclaim 17, wherein the transitional area comprises 0.1% to 20% of thethickness of the cable sheath.
 19. The optical cable of claim 17,wherein the gap has a thickness in the range of 45 μm to 55 μm and aninternal diameter of the protective sleeve is in the range of 800 μm to1200 μm.
 20. The optical cable of claim 15, further comprising a fillingcompound in the gap, the filling compound comprising a gel with aviscosity of greater than 6000 mPas at a temperature of 23° C.
 21. Amethod for producing an optical cable, comprising: providing atight-buffered optical cable; extruding a protective sleeve around thetight-buffered optical cable; surrounding the protective sleeve with anintermediate layer which has tension-resistant elements; andpressure-extruding a cable sheath around the intermediate layer suchthat the material of the cable sheath is mixed with the material of theintermediate layer in a transitional area.
 22. The method of claim 21,wherein pressure-extruding comprises: providing the material which formsthe cable sheath; surrounding the intermediate layer with the materialof the cable sheath such that contact is made; pressing the material ofthe cable sheath together with the intermediate layer, which surroundsthe protective sleeve, through a common opening whose diametercorresponds essentially to the external diameter of the optical cableafter cooling down; and cooling down the cable.
 23. The method of claim22, wherein the common opening has a diameter of about 5 mm.
 24. Themethod of claim 21, wherein the material of the cable sheath comprises aflame-retardant non-corrosive material mixture.
 25. The method of claim24, wherein the material of the cable sheath comprises polyethylene withaluminum trihydroxide or magnesium hydroxide.
 26. The method of claim21, wherein pressure-extruding comprises: providing a material whichforms the protective sleeve; surrounding the tight-buffered opticalcable with the material of the protective sleeve; stretching thematerial of the protective sleeve; and cooling down the protectivesleeve.
 27. The method of claim 26, wherein the material of theprotective sleeve is stretched until it makes contact with thetight-buffered optical cable.
 28. The method of claim 26, wherein thematerial of the protective sleeve is stretched until the gap between thetight-buffered optical cable and the protective sleeve is in the rangefrom 40 μm to 60 μm.
 29. The method of claim 21, wherein pressureextruding the protective sleeve comprises: providing a filling material;surrounding the tight-buffered optical cable with a layer composed ofthe filling material.
 30. The method of claim 29, wherein the fillingmaterial has a viscosity which is greater than 6000 mPas.
 31. The methodof claim 29, wherein a thickness of the filling material is in the rangefrom 40 μm to 60 μm.
 32. The method of claim 29, wherein the fillingmaterial is a gel.
 33. The method of claim 21, wherein the protectivesleeve comprises mixture of polycarbonate andacrylonitrile-butadiene-styrene.
 34. The method of claim 21, wherein theintermediate layer comprises a yarn including aramide fibers.
 35. Themethod of claim 21, wherein the intermediate layer comprises glassfibers.
 36. The method of claim 21, wherein an external diameter of thetight-buffered optical cable is about 900 μm.